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It's always emphasized in literature that endothermy enables high growth rates but requires more food.

But look at frogs, for example: bullfrogs, toads, pacman frogs etc. They would readily eat a quarter their weight a day, and they grow real fast (I used to keep pet frogs: pacmans, for example, become more that twice as big in one season). And all of that despite being ectotherms. How is that possible?

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Rather than saying ectotherms can't grow fast (which is obviously false), it's probably better to think about it as a two-part argument:

  1. Ectotherms and endotherms can grow rapidly at an optimum temperature
  2. Ectotherms can't maintain themselves at their optimum growth temperature

If you keep ectotherms at their optimum temperature, then they can reach optimum growth. In the wild, they might not find that optimum temperature for more than a few hours per day, though in your aquarium they might be at it all day long.

There are several publications supporting this such as

I don't see a specific reference for this but will add that ectotherm probably have greater growth potential than endotherms if they're at their optimum temperature, because endotherms have to waste energy on a whole suite of molecular machinery that's designed to maintain their body temperature. All that extra energy can be diverted into growth by ectotherms.

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  • $\begingroup$ Well, they don't grow slow in wild either; besides, amphibians in general prefer much lower temperatures (they don't bask like reptiles, and prefer something from 18 to 24 C). So basically, their metabolism just works differently? "Ectotherms and endotherms can grow rapidly at an optimum temperature" Well, the way I thought it is, in endotherms, their cells conduct reactions faster, which allows them to grow faster - and consequently, faster reactions produce more heat (basicelly, ectoterms grow faster and hence they are warmer, not the other way around). Thanks for references! $\endgroup$
    – A.V. Arno
    Dec 11, 2018 at 18:25
  • $\begingroup$ Frogs don't bask like reptiles? But it's common to see them out of the water, sitting on rocks & logs. $\endgroup$
    – jamesqf
    Dec 11, 2018 at 18:43
  • $\begingroup$ Well, some of them may have such a habit. But many don't, and many are nocturnal in the first place. $\endgroup$
    – A.V. Arno
    Dec 11, 2018 at 18:58

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